Fault current limiters are used to provide protection against current surges, for example in a power transmission network. Superconducting Fault Current Limiters (SCFCL) are a class of devices that operate at a cryogenic temperature and are typically used in electrical transmission or distribution lines that are subjected to high voltages and high currents. In a resistive SCFCL, the current passes through the superconductor component of the SCFCL such that when a high fault current begins, the superconductor quenches. In other words, the superconductor becomes a normal conductor where the resistance rises sharply and quickly.
A core of a SCFCL device may consist of several superconducting elements that are interconnected in series and parallel using non-superconducting connectors, which may dissipate power and increase cryogenics thermal load. In a normal operating mode, the SCFL device is cooled to cryogenic temperatures in order for the superconducting elements, such as tapes, to enter the superconducting state. When a current surge takes place along a transmission line, the current may enter the SCFCL, at which point it travels through the superconducting elements. If the current surge exceeds a critical value of current density in the superconducting tapes, the superconducting material may transform into a normal conductor (i.e. quench). Once in the normal conducting state, the superconductor material acquires a finite resistance to current which may limit the current conducted through the SCFCL to acceptable levels, thereby regulating the current conducted along the transmission lines.
In conventional SCFCL systems, the SCFCL presents nearly-zero impedance to a current load during normal operation, and inserts a large limiting impedance in the event of a fault condition such as, for example, a short circuit in order to reduce the fault current. An additional requirement for proper operation of an SCFCL is that after a fault condition is cleared, the SCFCL recovers rapidly within seconds to its former superconducting state in order to limit current in other fault events that may occur.
Currently, a major remaining challenge is the ability of an SCFCL to recover when a load current is carried through the SCFCL during a recovery period. SCFCLs that are under active development include, among others, systems using magnesium diboride wire, Yttrium Barium Copper Oxide (YBCO) tape, or Bismuth Strontium Calcium Copper Oxide (BSSCO) materials, which are cooled to below their respective superconducting transition temperatures (Tc) in order to function as designed. YBCO and BSSCO-based devices are attractive because the Tc in typical commercial materials is in the range of 90°-105° K, allowing SCFCL devices to operate using relatively inexpensive liquid nitrogen or boiling nitrogen cooling. However, when an SCFCL such as a YBCO or BSSCO system quenches and enters a state of finite resistance during a fault, the persistence of load current drawn through such an SCFCL may be problematic. In particular, load current drawn through YBCO or BSSCO (or other) elements that have a finite resistance may result in unwanted heating of the superconductor elements. Even a modest current drawn through the SCFCL may delay the return of the superconductor material to its superconducting state, thereby compromising performance of the fault limiting system. In a worst case, the SCFCL system may not recover at all and the superconducting elements of the SCFCL may remain as normal state conductors. It will be apparent therefore that improvements are desirable over known SCFCL systems.